1. Field of the Invention
This invention relates to optical flip chips, and more particularly to large bumps between a die and a substrate with a height greater than or equal to a height of a waveguide between the die and substrate.
2. Background of the Invention
FIG. 1 is a side cross sectional view of an optical device assembly 100. There is a substrate 102 connected to a die 106 by conductive bumps 110. The conductive bumps 110 are gold and formed by a wire bonder. To form the conductive bumps 110, gold wire is first attached to the die 106 under ultrasonic power, heat and force, then the wire is flattened to form the bumps 110. There is a gap 108 between the substrate 102 and the die 106. The height of the gap 108 is defined by the height of the bumps 110 that connect the substrate 102 to the die 106 and separate the substrate 102 and die 106 to create the gap 108. The gap 108, and thus the height of the bumps 110, is about 70 micrometers or less.
There is a waveguide 104 between the substrate 102 and the die 106. The waveguide 104 has a height 116 of about 100 micrometers or more. Since the gap 108 between the substrate 102 and die 106 is about 70 micrometers or less, and the height 116 of the waveguide 104 is about 100 micrometers or more, there is a trench 112 in the substrate 102 in which the waveguide 104 is located. The trench 112 is a cavity that extends below the top surface of the substrate 102. The trench 112 has a depth 114 deep enough so that the depth 114 of the trench 112 plus the height of the conductors 110 is greater than or equal to the height 116 of the waveguide 104. This trench 112 allows the waveguide 104 to fit between the substrate 102 and die 106 even though the gap 108 between the substrate 102 and die 106 is not as large as the waveguide 104.